Durability of offset printing plates



United States Patent 3,378,372 DURABILITY 0F OFFSET PRINTING PLATES Camille Angelina Vandeputte, Mortsel-Antwerp, Albert letrus Wagemans, Schoteu, and Arthur Henrica De Roeck, Edegem, Belgium, assignors to Gevaert Photo- Producten N.V., Mortsel, Belgium, a Belgian company No Drawing. Contiuuation-in-part of application Ser. No. 364,838, May 4, 1964. This application May 11, 1966, Ser. No. 549,137 lairns priority, application Belgium, May 2, 1963, 42,591, Patent 631,790 Claims. (Cl. 96-29) ABSTRACT OF THE DISCLOSURE A planographic printing plate carrying silver metal image (printing) areas obtained by diffusion transfer from an exposed silver halide photographic emulsion is improved as to durability by coating in the presence of water with a lithographic lacquer containing an alkyl resin alone or in admixture with a phenol-formaldehyde resin, which lacquer selectively adheres to the silver image areas. The lacquer may additionally contain a drying oil and is preferably dried by heating. An optional lacquer has the resin solution dispersed in an external aqueous phase.

This invention relates to a method for improving the durability of an offset printing plate having a hydrophilically-oleophilically differentiated surface, and to this improved offset printing plate itself.

This is a continuation-in-part application of the United States patent application Ser. No. 364,838, now abandoned.

Materials comprising a hydrophilically-oleophilically differentiated surface can be manufactured in many ways, e.g. by image-wise exposing and hardening a light-sensitive layer comprising a hardenable hydrophilic colloid carried by a hydrophilic support and by eliminating the unhardened areas, thus producing an oleophilic image on a hydrophilic support.

A further method for obtaining a hydrophilicallyoleophilically differentiated surface is described in the British patent specification 913,591. According to the process described in said British patent specification a light-sensitive material comprising a silver halide emulsion layer is exposed image-wise and subsequently developed. The unexposed and consequently undeveloped silver halide is complexed and transferred to a metal layer on which silver is image-wise precipitated from the formed complexes through the reducing action of the metal layer itself. The precipitated silver is found to be oleophilic. The metal layer mainly consists of hydrophilic metal or metal which can easily be made hydrophilic, such as very pure aluminum or Zinc containing minor amounts of other metals such as silicon, iron and copper and occasionally containing slight amounts of impurities.

In theory the thus obtained materials could be used as an offset printing form as well. Yet, in practice one never uses such untreated offset printing plates for printing since the differentially hydrophilic-oleophilic nature thereof uses to be insufficiently pronounced and because the oleophilic printing areas lack for the strength that should enable the offset printing plates to supply a high number of copies.

As a rule, the offset printing plate is treated with an etching solution which comprises components improving the hydrophilic properties of the non-printing areas and often also contains components which improve the elecphilic properties of the printing areas as well. Such etching solutions are described e.g. in the British patent specification 946,538, disclosing the same subject matter as U.S. Patent 3,300,306, issued Jan. 24, 1967.

Generally there is also applied a lacquer to the image- Wise hydrophilically-oleophilically differentiated surface in order to strengthen the printing parts of the lith0- graphic printing plate.

It is known from the United States patent specification 1,951,943 to apply a phenolic condensation product selectively to the hydrophobic areas of a moistened hydrophilically-oleophilically differentiated lithographic plate.

It is known from the British patent specification 968,706 to apply a phenol-formaldehyde resin from its solution in an organic solvent selectively to the hydrophobic areas of a printing plate obtained according to the method described in the said British patent specification 913,591.

Finally, it is known from the Belgian patent specification 599,934 and its patent of addition 620,344 to apply a lacquer which simultaneously improves the hydrophilical-oleophilical nature of the surface of a lithographic plate and strengthens the printing areas. The lacquer consists of both a continuous phase and a dispersed phase. The continuous phase contains water, a thickening agent and occasionally phosphoric acid, whereas the dispersed phase comprises a solution of a phenol-formaldehyde resin in an organic solvent that is immiscible with Water.

The phenol-formaldehyde resins known from the above patent specifications are generally sufficiently selective i.e. they deposit onto the oleophilic areas of the offset printing plate and not onto the hydrophilic areas. Usually, however, they are found not to possess suflicient structural strength, i.e. they generally form only a rather thin membrane on the oleophilic areas of the offset printing plate, so that the plate starts wearing off considerably after the printing of a few thousands of copies and soon becomes unfit for use.

It has now been found that the durability of an offset printing plate comprising hydrophilic areas and oleophilic areas corresponding to the image to be printed, can be improved considerably and that an offset printing plate of an excellent quality can be obtained by applying to the said surface comprising hydrophilic and oleophilic areas a lacquer coating composition comprising at least one alkyd resin or a lacquer coating composition comprising at least one alkyd resin together with at least one phenol-formaldehyde resin, this lacquer coating composition being applied in the presence of water, supplied directly in a preliminary step and/or indirectly by applying a two phase lacquer coating composition one phase of which is an aqueous phase.

According to the method of the invention the alkyd resin alone as well as the alkyd resin and the phenolforrnaldehyde resin both applied together show an excellent selectivity of depositing onto the oleophilic or printing areas only of the offset printing plate, which selectivity in many cases is better than when a phenolformaldehyde resin alone is applied. Moreover they mostly possess an improved structural strength over the phenol-formaldehyde resins alone.

From the United States patent specification 2,258,956 it is already known to use phenol-formaldehyde resin together with a drying oil acid modified glycerol phthalate resin for strengthening the printing areas of a lithographic plate. However, according to said patent specification the lacquer is applied to the lithographic plate whilst the latter is dry whereby the resins deposit onto the printing areas as well as the non printing areas, from which latter areas they are removed later on whereafter still a desensitizing solution including gum arabic and chrome alum is applied to the non printing area of the plate in order to improve the hydrophilic or ink repellent properties of these areas. Regardless the fact that the method described in the United States patent specification 2,258,956 is quite cumbrous it is not obvious from the teaching of this specification that an alkyd resin alone or together with a phenol-formaldehyde resin when applied to a hydrophilically-oleophilically differentiated lithographic pate in the presence of water deposits so selectively and with such good structural strength onto the oleophilic or printing parts of the plate only.

The water in the presence of which the lacquer coating composition is applied to the hydrophilicallyoleophilically differentiated surface may be supplied directly by a preliminary treatment of the plate with a composition containing water, preferably with pure water or with an aqueous solution such as the etching solution for improving the hydrophilic properties of the non-printing areas and often also improving the oleophilic properties of the printing plate. However, the water may also indirectly be supplied by the lacquer coating composition itself when the latter is a two-phase composition one phase of which is an aqueous phase. In case such a two-phase lacquer composition is applied water may still be supplied as well by a preliminary treatment as described.

The application of the lacquer to the hydrophilicallyoleophilically differentiated surface (further on in the description designated as differentiated surface) is very simple and in practice generally comprises pouring some lacquer over the differentiated surface and rubbing out this lacquer over the surface by means of a plug of wadding or the like. The lacquer is accepted selectively by the olephilic areas, which result to be the printing areas of the offset printing plate.

Alkyd resins, which are particularly appropriate for being used in the preparation of the lacquer, are synthetical resins resulting from the polycondensation of at least one polyfunctional alcohol with at least one polyfunctional acid, occasionally admixed with or replaced by a hydroxycarboxylic acid, and occasionally also with at least one saturated or unsaturated monobasic acid. In other words, pure alkyd resins as well as modified alkyd resins are suited. For convenience the expression alkyd resins will be used in the description and in the claims, but pure as well as modified alkyd resins are meant thereby. Solid alkyd resins are preferably utilized. Liquid alkyd resins are equally very selective, but they are only efficient if the final offset printing plate is subjected to a heat treatment yet. Appropriate polyols are i.a. ethylene glycol, glycerol, and pentaerythritol. Suited polybasic acids are i.a. .terephthalic acid, isophtalic acid, orthophthalic acid, abietic acid, adipic acid, sebacic acid, trimellitic acid, maleic acid and particularly rosin containing 80 to 90% abietic acid and its anhydride. Occasionally the anhydride may be utilized in lieu of the acid. Unsaturated or saturated monobasic acids suitable for the preparation of a suited modified alkyd resin are illustrated by the unsaturated or saturated fatty acids. For more particulars about alkyd resins there reference may be had to C. R. Martens, Alkyd Resins, Reinhold Publishing Corp, New York, 1961. All polyesters are polycondensation products of at least one poly-functional acid with at least one polyfunctional alcohol. If they are solid and soluble in the common organic solvents, which are at most slightly water-miscible, such as cyclohexanone, pentanone, butyl acetate and amyl acetate, polyesters other than alkyd resins are generally also suited for being used either together with or in lieu of the described alkyd resins. The matter described in the present application as to the use of alkyd resins may consequently be applied to the above polyesters just as well.

A lacquer for being used in the present method can be a solution of at least one alkyd resin in a suited organic solvent, which is preferably not too volatile and which is not or only slightly miscible with water, such as an aliphatic ketone e.g. methyl isobutyl ketone, di-

ethyl ketone, and ethylbutyl ketone, an ester of an aliphatic acid and an aliphatic alcohol e.g. amyl acetate, and a cycloaliphatic solvent e.g cyclohexanone, and mixtures of these solvents, etc.

The concentration of alkyd resin in such solution depends on so many variables, such as the nature of the resin, the nature of the organic solvent, admixtures occasionally present in the lacquer, the nature of the differentiated surface to be lacquered, etc., that this concentration may vary within very wide limits. In practice, however, this concentration usually ranges from 20 to 100% by weight per volume of lacquer.

Besides the alkyd resin the lacquer solution preferably also contains at least one resin of the phenolformaldehyde type. More particulars about these resins can be found in Methoden der organischen Chemie, Houben-Weyl, 4. Auflage, Band 14-2, pp. 193-291, Thieme Verlag, Stuttgart, 1963.

Suitable phenol-formaldehyde resins are described also in the British patent specification 968,706. Lacquers containing alkyd resins as well as phenol-formaldehyde resins are particularly suited since they consolidate the oleophilic areas of an offset printing plate with a layer constructed as good as an alkyd resin layer and which moveover tends to be harder, so that an excellent protection of the printing areas is attained. If the lacquer contains alkyd resin as well as phenol-formaldehyde resin, the total concentration of resin is comprised preferably within the same limits as if the lacquer would contain the sole alkyd resin. In a lacquer containing both types of resin, the concentration of phenolformaldehyde resin may vary from less than 1% by weight to by weight of the total weight of resin. This, however, is dependent on a great many variables involved.

In a lacquer as described above there may be dissolved or dispersed other components yet. For instance a pigment may be dispersed in the lacquer in a concentration preferably comprised between 5 to 20% by weight of the total volume of lacquer. The presence of pigment enhances the more selective deposition of the solid components of the lacquer on the oleophilic areas of the differentiated surface. Moreover, the incorporation of pigment in the lacquer composition offers the advantage that the lacquered printing areas of the offset printing plate are coloured and thereby become more visible. The latter effect may also be attained by dissolving organic dyes in the lacquer. Pigments suited for dispersal in the lacquer composition are i.a. toluidine red, vine black, chrome black, etc. Care should be taken, however, that the grain of the pigment is not too coarse.

There is very often added to the lacquer a drying oil such as linseed oil or castor oil, and/ or a so-called polymeric plasticiser such as a liquid low molecular weight polyester, usually in a concentration of 10 to 30% by volume of the total volume of lacquer. This way, the inkreceptive properties of the oleophilic areas of the offset printing plate are further improved and the solid components of the lacquer will even more selectively deposit on these areas.

The best results are attained with lacquers containing an alkyd resin, a phenol-formaldehyde resin, a pigment, and a drying oil. If these components are present in a suited proportion in the lacquer, it is possible with the aid of such lacquer to obtain an offset printing plate by which more than 50,000 copies of even and excellent quality can be printed. Usually the resin (alkyd and occasionally the phenol-formaldehyde resin) is dissolved first in the organic solvent or in the mixture of organic solvents. Subsequently the pigment is dispersed therein whilst thoroughly stirring. Finally the drying oil is added slowly and mixed carefully with the dispersion.

Lacquers as described above could be designated as organic lacquers. It is possible also, however, to apply in a process according to the present invention a two-phase lacquer one phase of which is an aqueous phase. Such a two-phase lacquer may be a dispersion of an organic lacquer as described above in an aqueous phase. The volume proportion of the organic phase to the aqueous phase is preferably between and /5. Such organicinorganic lacquer composition has the desirable capacity of making it generally and principally superfluous to supply water by apreliminary step and or to be able to contain in its aqueous phase components which otherwise must be applied by way of the etching solution, so that the use of a lacquer composition of this type does away with the need of treating the differentiated surface with a separate etching solution before the application of the lacquer.

The acqueous phase may consist of water alone, but in addition thereto should contain a certain amount of an emulsifying agent in order to allow the formation of a stable dispersion of the organic lacquer composition in this aqueous phase. Suited emulsifying agents are sodium lauryl sulphate, alkaline salts of sulphonated dicarboxylic acid derivatives such as dioctyl sodium sulphosuccinate, and alkaline salts sulphonates of saturated hydrocarbons.

In this aqueous phase may be dissolved components improving the hydrophilic properties of the non-printing areas of the differentiated surface, such as phosphoric acid, water-soluble primary and secondary phosphates, and thickening agents such as carboxymethylcellulose, gum arabic, sodium alginate, poly(vinyl alcohol), poly(acrylic acid), poly(methacrylic acid), poly(styrene sulphonic acid), and poly(vinyl pyrrolidone) preferably in a concentration varying from 2 to by weight per volume of lacquer composition.

The aqueous phase may also comprise components, which improve the oleophilic properties of the differentiated surface. If the oleophilic areas mainly consist of metallic silver, suitable components are cationic surfaceactive compounds, compounds containing a mercapto group, combinations of these two kinds of compounds, etc. For more particulars the British patent specification 946,538 may be referred to.

To the aqueous phase may be added also bactericides, which should prevent the degradation of some thickening agents by enzymes. Suitable bacteridices are pentachlorophenol, formaldehyde, and phenol.

The preparation of the aqueous 'phase is very simple since it usually only consists of dissolving the abovementioned compounds in water. The sequence in which the compounds are added is usually of minor importance.

The organic-inorganic lacquer composition may be prepared from the organic lacquer and the aqueous phase by dispersing the organic phasein the aqueous phase whilst thoroughly stirring.

Although the process according to the present invention is suited for being applied to any hydrophilically-oleophilically differentiated surface, it is especially suited for being applied to a surface, which has been differentiated according to the process described in the introduction of the present application. The principle of this process is the subject matter of the British patent specificatio 91-359 1.

Finally it should be mentioned that in many cases the quality of the lacquered offset printing plate may still be improved by heating. This may occur in an oven at a temperature preferably comprised between 100 and 300 C. Heating within the above temperature range is recommended especially when the method according to the present invention is carried through with the use of drying oil modified alkyd resins.

The following examples illustrate the invention.

Example 1 Example 1 of British patent specification 946,538 is repeated as follows:

To a silver chloride emulsion ready for coating, is added CdClgZHgO in such a manner and in such an amount that, after coating, the emulsion layer contains per sq. m. 1.43 g. of silver and 0.51 g. of cadmium. The emulsion is coated on a baryta coated paper base and the light-sensitive paper thus obtained is image-wise exposed. The exposed negative and an aluminium sheet consisting of 99.5% of aluminium and 0.5% of magnesium and silicon and the surface of which is brushed to grain depths of 2 to 4 and are passed through a usual apparatus for carrying out the diffusion transfer process. This apparatus contains an aqueous developer solution of the following composition:

G. Sodium hydroxide 11 Sodium sulphite (anhydrous) Sodium th-iosulph ate (anhydrous) 6 Potassium bromide 1.5 Hydroquinone 9 l-phenyl-3-pyrazolidone 1.5 Ethylene diamine tetra acetic acid trisodium salt 4 Water to 1000 cmfi.

G. Canboxymethylcellulose 3.24 Sodium phosphate 0. 6 Phosphoric acid 0:3

Cetyl trimethyl ammonium bromide 0.06 Aqueous solution of formaldehyde 20% 1 Water to 100 cm.

The lithographic printing plate thus obtained possesses a hydrophilicallyoleophil-ically differentiated surface. This material is even more suited for being used as an offset printing plate if a lacquer is applied thereto with the aid of a plug of wadding at the moment that the plate is still wet from the application of the aqueous lithographic preparation, said lacquer having the following composition:

Methyl isobutyl ketone ccs 100 Alkyd resin g '85 Waxoline Rhodamine BS (Cl. 45, B) g 3 The alkyd resin used is a pentaerythritol ester of Wood rosin having a melting point between 109 and 116 C. and an acid number of at most 16.0.

Example 2 Example 13 of British patent specification 946,538 is repeated as follows:'

A baryta coated paper base is provided with a silver chloride emulsion containing 0.35 g. of zinc per sq. m. of paper is image wise exposed and developed in the presence of an aluminium sheet in an aqueous developer solution as in Example 1. After passing through that solution and between a pair of rubber rollers so that the light-sensitive paper and the aluminium sheet are pressed together, they are separated from each other. Immediately thereafter, the sheet bearing the silver image formed is rubbed for about 30 seconds with a cloth soaked with a lithographic preparation of the composition as set forth 7 in Example 1, the 0.06 g. of cetyltrimethylamrnonium bromide of which having been replaced by the same quantity of 1-phenyl-5-mercaptotetrazole.-

The material thus obtained possesses a hydrophilical-lyoleophilically differentiated surface and is even more suitable for being used as an offset printing plate if a lacquer is applied thereto with the aid of a plug of wadding at the moment that the plate is still wet from the application of the aqueous lithographic preparation, said lacquer having the following composition:

Cyclohexanone ccs 100 Alkyd resin g 50 Castor oil ccs 1'20 Bleu Marin Orasol RB (Cl. Solvent Blue 1) g 4 The alkyd resin used is a pentaerythritol ester of rosin having a melting point between 130 and 137 C. and an acid number between 30 and 40.

Example 3 An aqueous solution of 40 g. Cd(NO 41-1 is added to 1 kg. of a silver chloride iodide emulsion (0.6% iodide), ready for coating, and containing 25 g. silver per kg. The emulsion is coated on a baryta coated paper base. This paper is treated as in Example 1. Immediately after development and separation of the light-sensitive paper from the aluminium sheet, the latter is rubbed for about 30 seconds with a cloth soaked with the same lithographic preparation as in Example 1 but to which 0.06 g. of l-phenyl--mercaptotetrazole has been added.

The material thus obtained possesses a hydrophilicallyoleophilically differentiated surface and is even more suited for being used as an offset printing plate if a lacquer is applied thereto with the aid of a plug of wadding at the moment that the plate is still Wet from the application of the aqueous lithographic preparation, said lacquer having the following composition:

Diethyl ketone ccs 50 Alkyd resin ccs 50 Toluidine red R (Cl. 12, 120) g The alkyd resin used in this composition is a polyester of the alkyd type manufactured by the C. P. Hall Company, Chicago, Ill., U.S.-A. under the trade name Polymeric Plasticizer HALLCO HA-S-Af The molecular weight is aproximately 4,200, the viscosity at 99 C. approximately 290 0ps., the acid number is at most 3,0 and the saponifioation number ranges from 439 to 447.

The resulting offset printing plate is then heated for 10 minutes in a furnace at 200 C.

Example 4 The process of Example 2 is repeated, but the lacquer composition is replaced by the following composition: Cyclohexanone ccs 100 Alkyd resin g 30 Mikro-Rebenschwarz MM 170 (trade name of a vine Black brand marketed by Ari-Chemie, Vockenhausen/Taunus, W. Germany) g Linseed oil ccs The alkyd resin used in a rosin maleic acid ester manufactured by Jones and Dabney, Newark, NJ., U.S.A., under the trade name Syntex H3. The alkyd resin manifests a melting point between 280 and 300 F. and an acid number between 30 and 40. More particulars about this alkyd resin can be found in the publication Resins and Chemicals (1957) of Jones and Dabney.

Example 5 The process of Example 1 is repeated, but with the following lacquer composition:

Methyl isobutyl ketone ccs 100 Alkyd resin g 20 Phenol formaldehyde resin g 25 Waxoline Rhodamine BS g 3 The alkyd resin used in this composition is identical to that used in Example 1. The phenol-formaldehyde resin is a resin originating from the reaction of surplus phenol with formaldehyde in acid medium. The meltiw point of this phenol-formaldehyde resin is comprised between 100 and 105 C. and the viscosity of a solution of 50 g. of this resin in 50 cos. of ethanol varies from 60 to 70 cps. at 35 C.

Example 6 The process of Example 5 is repeated but with the following lacquer composition:

The alkyd resin and phenol-formaldehyde resin used are those employed in Example 5.

Example 7 The process of Example 3 is repeated, but with a lacquer having the following composition:

Diethyl ketone ccs 50 Alkyd resin ccs 25 Phenol-formaldehyde resin g 15 Toluidine red R g 10 The alkyd resin used is the one used in Example 3. The phenol-formaldehyde resin used is a o-cresolformaldehyde resin, the preparation of which is described in the British patent specification 968,706.

Example 8 The process of Example 7 is repeated but with a lacquer having the following composition:

Cyclohexanone ccs 75 Alkyd resin g 12 Phenol-formaldehyde resin g 10 I Linseed oil ccs I18 Toluidine red R g '12 The alkyd resin used is that employed in Example 4. The phenol-formaldehyde resin used is that of Example 7.

Example 9 The following solutions are prepared:

Solution A:

20% aqueous solution of gum arabic ccs 50 Sodium salt of carboxymethylcellulose g 1 40% aqueous solution of formaldehyde cc 1 Sodium lauryl sulphate g 2 Solution B:

Methyl isobutyl ketone ccs 20 Alkyd resin g 4 Phenol-formaldehyde resin g 5 Waxoline Rhodamine BS g 2 The carboxymethylcellulose used displays a degree of substitution from 0.65 to 0.85 and the viscosity of a 2% aqueous solution varies from 25 to 50 cps. at 25 C.

The alkyd resin and phenol-formaldehyde resin used are those used in Example 5.

Solution B is dispersed in solution A whilst thoroughly stirring the latter solution.

A plug of wadding soaked with this dispersion is rubbed out on an aluminium foil carrying a silver image obtained according to Example 1 above, which foil has not been treated yet with an etching liquid. I-t this way a lacquered offset printing plate of excellent quality is obtained.

Example 10 The following compositions are prepared:

Solution A:

2% aqueous solution of the sodium salt of carboxymethylcellulose ccs 50 Monosodium phosphate g 0.5 Pentachlorophenol g 0.1 Sodium lauryl sulphate g 2 9 Composition B:

Diethyl ketone ccs 5 Alkyd resin ccs 2.5 Phenol-formaldehyde resin g 1 Toluidine red R g 2 The carboxymethylcellulose used is that of Example 9. The alkyd resin and phenol-formaldehyde resin used are those employed in Example 7.

The composition B is dispersed in solution A whilst thoroughly stirring the latter solution.

The process of Example 9 is then repeated, but with the above-described dispersion instead of the dispersion used in Example 9.

Example 11 The following compositions are prepared:

Solution A:

5% aqueous solution of the sodium salt of poly (styrene sulphonic acid) ..ccs 50 Monosodium phosphate g 0.5 1-phenyl-5 mercaptotetrazole g 0.1 Cetyldimethylbenzoylammonium chloride g 0.2 Sodium lauryl sulphate g 2 Composition B:

Cyclohexanone L ccs 8 Alkyd resin g 1.2 Phenol-formaldehyde resin g 1 Linseed oil ccs 2 Toluidine red R g 1.2

The poly(sty=rene sulphonic acid) used has been prepared by sulphonating polystyrene having a molecular weight of approximately 5,000 in such a way that one sulphonic acid group is present per benzene nucleus.

The alkyd resin used is that used in Example 4 and the phenol-formaldehyde resin used is that of Example 7.

The process of Example 9 is then repeated, but with the above-described dispersion instead of that of Example 9.

What we claim is:

1. In a method of improving the durability of an olfset printing plate obtained by the steps comprising contacting a plate having at least a surface layer of a hydrophilic metal with a light-sensitive silver halide material, including a silver halide emulsion layer, carrying in the silver halide emulsion layer a latent photographic image formed by exposure to light, in the presence of an aqueous alkaline processing liquid, a photographic developing agent for said emulsion and a silver halide complexing agent to solubilize said silver halide, whereby said complexed silver halide diffuses from the unexposed areas of said emulsion layer onto the corresponding areas of said hydrophilic metal surface where it is reduced to precipitate a layer of silver metal on said corresponding areas to serve as a printing image, the improvement which comprises the steps of applying to said plate a solution in an organic solvent substantially immiscible in water of at least one alkyd resin, this solution being applied in the presence of water, and evaporating the solvent from said solution film to deposit onto said oleophilic areas a resin coating of durable oleophilic character.

2. The method of claim 1, wherein the Water is supplied by a preliminary treatment of said plate With a composition containing water.

3. The method of claim 1, wherein said solution is of at least one phenol-formaldehyde resin in addition to said alkyd resin, said phenol-formaldehyde resin being present in the amount of 180% by weight of the total resin present in said solution.

4. The method of claim 3, wherein said solution contains at least one drying oil in addition to said resins, said drying oil constituting about 10-30% by volume of said solution.

5. The method of claim 1, wherein said solvent is evaporated from said solution film by subjecting th plate bearing said film to a heat treatment.

6. The method of claim 1, wherein said solution is dispersed in an aqueous phase.

7. The method of claim 6, wherein said solution has dissolved therein at least one phenol-formaldehyde resin in addition to said alkyd resin, said phenol-formaldehyde resin being present in the amount of 180% by weight of the total resin present in said solution.

8. The method of claim 7, wherein said solution additionally contains at least one drying oil, said drying oil constituting about 1030% by volume of said solution.

9. The method of claim 6, wherein the solvent is evaporated from the solution film by subjecting the plate bearing the film to a heat treatment.

10. The method of claim 1 wherein the hydrophilic metal surface actively takes part in the reduction of the diffusing complexed silver halide, whereby metallic silver is deposited from said complexed silver halide onto said metal surface.

References Cited UNITED STATES PATENTS 1,951,943 3/1934 Mossberg l0ll49.2

2,258,956 10/1941 Misuraca 101-1492 2,754,279 7/1956 Hall 101-1492 X DAVID KLEIN, Primary Examiner.

ROBERT E. PULFREY, Examiner.

J. A. BELL, Assistant Examiner. 

